why the millennium bridge rocked

London's Millennium Bridge was a fiasco: once it opened, pedestrians walking across it set it rocking dangerously, and it was quickly closed and retrofitted. Today engineer Stephen Strogatz explains how this happened, in the journal Nature.

He says people stepping in unison caused vibrations, which then made people walk in a funny way, in resonance with the bridge, which then made the wobbling worse. But it's a chicken-and-egg problem: which came first, the synchronous stepping or the bridge's bending? Strogatz concludes that all it would take is a few people to randomly fall into lockstep, and that would set off a feedback loop that made the bridge dangerous.

Link to Newswise press release